Synopsis: Conditions are favorable for a transition from ENSO-neutral to El Niño conditions during June - August 2009.

ENSO-neutral conditions persisted across the equatorial Pacific Ocean during May 2009. However, sea surface temperatures (SST) increased for the fifth consecutive month, with above-average temperatures extending across the equatorial Pacific Ocean by the end of May (Fig. 1). Accordingly, the latest weekly SST indices ranged between +0.4o to +0.5°C in all four Niño regions (Fig. 2). Subsurface oceanic heat content anomalies (average temperatures in the upper 300m of the ocean, Fig. 3) also continued to increase in response to a large area of above-average temperatures (+2° to +4°C) near thermocline depth (Fig. 4). These surface and subsurface oceanic anomalies typically precede the development of El Niño.

From early 2007 through April 2009, enhanced low-level easterly winds persisted near the Date Line, interrupted only briefly by Madden-Julian Oscillation (MJO) activity. However, during May 2009, both the lower-level equatorial winds were near-average in that region despite the absence of the MJO. Also, suppressed convection expanded westward along the equator from the Date Line to Indonesia. The recent oceanic and atmospheric anomalies are consistent with ENSO-neutral conditions, but also reflect the evolution towards a potential El Niño.

There continues to be considerable spread in the model forecasts for the Niño-3.4 region (Fig. 5). All statistical models predict ENSO-neutral conditions will continue for the remainder of 2009. However, most dynamical models, including the NCEP Climate Forecast System, predict the onset of El Niño during June - August 2009. Current observations, recent trends, and the dynamical model forecasts indicate that conditions are favorable for a transition from ENSO-neutral to El Niño conditions during June - August 2009.

This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web
site (El Niño/La Niña Current
Conditions and Expert Discussions). Forecasts for the evolution of El Niño/La Niña are updated monthly in the
Forecast Forum section of CPC's Climate
Diagnostics Bulletin. The next ENSO Diagnostics Discussion is scheduled for 9 July 2009. To receive
an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message
to: ncep.list.enso-update@noaa.gov.